The nephrotic syndrome affects both children and adults and may lead to renal failure and death. Abnormalities of podocyte structure and the glomerular polyanion associated with nephrotic syndrome are wellrecognized. The anionic charges on the apical surface of the podocyte are required to maintain podocyte structure and function. Podocalyxin is the major sialoglycoprotein present on the urinary space surface of podocytes and contains most of the protein bound sialic acid of the glomerulus. A reduction in the sialylation of glomerular proteins results in proteinuria, foot process effacement, and nephrotic syndrome in experimental animals. We have cloned and characterized the podocalyxin cDNAs from rabbit, human, and mouse and have cloned the mouse podocalyxin gene. Podocalyxin knockout mice have major defects in the glomerular filtration apparatus at birth with no podocyte foot process formation. We hypothesize that podocalyxin is required for the formation and maintenance of a functional glomerular filter and for the development of podocyte foot processes. To define the function of podocalyxin we have searched for and isolated a putative linker protein that binds to the C-terminal of podocalyxin. This linker protein has the potential to link podocalyxin to the actin cytoskeleton in a protein complex containing eznn and regulatory enzymes. We will define the interactions of podocalyxin in this protein complex and determine the function of the podocalyxin-linker complex in vivo. To test the hypothesis that podocyte podocalyxin is required for foot process formation we will determine if a podocyte-expressed rabbit podocalyxin transgene will restore podocyte foot process formation in the podocalyxin knockout mice. To define the role of the podocalyxin C-terminal binding motif in podocalyxin function we examine the podocytes of podocalyxin knockout mice expressing a rabbit podocalyxin transgene lacking the podocalyxin C-terminal binding motif. These studies will advance our understanding of the role of podocalyxin in the development, structure, and maintenance of the glomerular filter.